Recently, various mobile communications systems such as a cellular service system, PCS system, GMS system and Iridium service system using a satellite are available throughout the world. For example, in Korea, cellular service, PCS and CT-2 systems are commercially provided. Portable terminals used in the mobile communications systems have been developed and pushed for improvement in compactness, multifunction, lightweight and low power-consumption. An antenna functions to transmit and receive a signal between a terminal and a base station, and is a critical component determining communication quality of the terminal. Since performance of the antenna may vary depending on the shape and material of the terminal where the antenna is mounted, the antenna should be designed compatible with a model of the terminal in order to obtain an optimal performance thereof.
In general, for the purpose of bidirectional communications and convenient possession, a non-directional retractable antenna is used as an antenna for a terminal. An antenna of a commercially available terminal has a combined structure suitable for both a signal waiting state and a communications state to transmit and receive a linearly polarized signal with ease. There are largely two kinds of antennas, i.e., a helical antenna and a monopole antenna.
The helical antenna has a spiral configuration which protrudes from a top of the terminal and has an advantage in that it can communicate regardless of the orientation of the terminal.
The monopole antenna is used in an extended state for a high quality communication. The monopole antenna has a greater ability in a vertical orientation than the helical antenna, but theoretically cannot receive a signal in a horizontal orientation.
The performance of such an antenna depends on the shapes of the terminals on which it is mounted and a matching circuit is provided between the antenna and a duplex in order to compensate the difference in performance.
The helical antenna has a spiral structure with a physical resonant length of λ/2 and λ/4 which uses a connection element. The helical antenna also has a ground surface and an electric power supplying line.
In particular, a conventional helical antenna is a single band helical antenna implemented by using ceramic sheets of thickness of tens or hundreds of micrometer and forming a vertical via hole and a horizontal pattern in each of the sheets. The implemented antenna structurally exhibits a single band characteristic, and is therefore unable to operate in two or more different bands.
Specifically, the cellular service system and the PCS system use, e.g., 824˜894 MHz band and 1750˜1870 MHz band, respectively, with the center frequencies thereof spaced apart from each other by about 1 GHz, and the center frequencies are not in an integer time relationship with harmonics component thereof. Accordingly, the conventional antenna cannot be used in both the cellular service system and the PCS system using different frequency bands even though a matching circuit is employed thereto.
Attempts have been made to allow one terminal to operate in various communications systems using different frequency bands so that the terminal can be used throughout the world. In this case, the antenna and other components employed in such terminal for various communications systems should meet an electrical standard and operate in two or more frequency bands. The antenna should also be able to operate in next-generation mobile communications services such as IMT-2000 with a broader frequency band and in two or more different mobile communications service bands.